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Not including entities like Abeloth

IMHO, some candidates in no particular order:

Palpatine
Luke Skywalker
Anakin Skywalker
Obi Wan
Yoda
Krayt
Revan
Vitiate
The Outlander
Marka Ragnos
The Exile
Darth Bane

Good list. I'd probably add Jacen/Caedus and Plagueis

Chronologically.

Xendor - Appeared to have led the first major schism within the Jedi Order, among the first of the Jedi to tap into the dark side.

Jedi Exiles - Ajunta Pall, XoXaan, Muur, Dreypa and Sorzus survived the Hundred Year Darkness following Xendor's rebellion, and it is they who arrived on Korriban, where they would dominate the Sith species and give the order of Sith it's name.
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Vitkorion - Appears to have some kind of link/gave inspiration for the Thought Bomb thanks to the Nathema ritual, raised one of the most powerful Sith Empires in history and then destroyed it and the Jedi Order under Zakuul.

Revan - Provided major inspiration for the Rule of Two as well as the Thought Bomb, and also telepathically manipulated Vitiate into making major concessions during the galactic war. And much more.

The Outlander - The only one capable of stopping Vitkorion once and for all.
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Darth Bane - Should require little introduction, but to recap, destroyed the Sith Order and instituted the Rule of Two, which would go on to perform a nearly complete genocide on the Jedi Order and rule the entire galaxy.

Darth Tenebrous - Artificially engineered not only Plagueis' birth, but also his genetics, specifically his fascination/aptitude for midichlorian manipulation. This would result in the conception of Anakin Skywalker, as well as his fall to the dark side, as well as Plagueis seeking out Sidious.

Darth Sidious - Hopefully needs little introduction, but to recap, successfully destroyed the Jedi Order, anointed himself the most powerful being in the galaxy which he ruled over indisputably, and turned the prophecy-bound Chosen One to the dark side.
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Obi-Wan Kenobi - Prevented Anakin Skywalker from becoming more powerful than anyone could ever hope to stop, and also guided Luke on his path to become a Jedi - not to mention saving his life as an infant by killing Maul.

Anakin Skywalker - Was integral to Sidious' demise, as well as the security of the galaxy since his actions caused the death of the Ones - otherwise the Son would have escaped eventually. He also gave birth to Luke and Leia, who would go on to shape the future of the galaxy massively.

Luke Skywalker - Also played an integral role in the demise of Sidious, twice, saved the entire galaxy from the Yuuzhan Vong, not to mention Abeloth. Also gave birth to a line of Skywalkers who would once again be integral to the security of the galaxy and the freedom of it's inhabitants.
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Darth Krayt - Through sheer determination, built an order of Sith from nothing which would go on to annihilate the Jedi Order, and rule the entire galaxy save for a select few star systems, instituting possibly the most dominant Sith empire in recorded history. Also played a pivotal role in defeating Abeloth.

Karness Muur - Inadvertently saved Krayt's life from Wyyrlok.

Kol and Cade Skywalker - Through their combined actions, they would topple the Sith Empire led by Darth Krayt, essentially saving the galaxy once again. Kol in particular is responsible for fostering a positive relationship between the remnants of the Yuuzhan Vong and the wider galaxy, and Cade's birth, giving the galaxy potential for a happy ending.

Freedon Nadd
Ulic Qel-Droma
Exar Kun
Revan
Malak
Traya
Sion
Nihilus
The Brotherhood of Darkness

Originally posted by ILS
Chronologically.

Xendor - Appeared to have led the first major schism within the Jedi Order, among the first of the Jedi to tap into the dark side.

Jedi Exiles - Ajunta Pall, XoXaan, Muur, Dreypa and Sorzus survived the Hundred Year Darkness following Xendor's rebellion, and it is they who arrived on Korriban, where they would dominate the Sith species and give the order of Sith it's name.
-----

Vitkorion - Appears to have some kind of link/gave inspiration for the Thought Bomb thanks to the Nathema ritual, raised one of the most powerful Sith Empires in history and then destroyed it and the Jedi Order under Zakuul.

Revan - Provided major inspiration for the Rule of Two as well as the Thought Bomb, and also telepathically manipulated Vitiate into making major concessions during the galactic war. And much more.

The Outlander - The only one capable of stopping Vitkorion once and for all.
-----

Darth Bane - Should require little introduction, but to recap, destroyed the Sith Order and instituted the Rule of Two, which would go on to perform a nearly complete genocide on the Jedi Order and rule the entire galaxy.

Darth Tenebrous - Artificially engineered not only Plagueis' birth, but also his genetics, specifically his fascination/aptitude for midichlorian manipulation. This would result in the conception of Anakin Skywalker, as well as his fall to the dark side, as well as Plagueis seeking out Sidious.

Darth Sidious - Hopefully needs little introduction, but to recap, successfully destroyed the Jedi Order, anointed himself the most powerful being in the galaxy which he ruled over indisputably, and turned the prophecy-bound Chosen One to the dark side.
-----

Obi-Wan Kenobi - Prevented Anakin Skywalker from becoming more powerful than anyone could ever hope to stop, and also guided Luke on his path to become a Jedi - not to mention saving his life as an infant by killing Maul.

Anakin Skywalker - Was integral to Sidious' demise, as well as the security of the galaxy since his actions caused the death of the Ones - otherwise the Son would have escaped eventually. He also gave birth to Luke and Leia, who would go on to shape the future of the galaxy massively.

Luke Skywalker - Also played an integral role in the demise of Sidious, twice, saved the entire galaxy from the Yuuzhan Vong, not to mention Abeloth. Also gave birth to a line of Skywalkers who would once again be integral to the security of the galaxy and the freedom of it's inhabitants.
-----

Darth Krayt - Through sheer determination, built an order of Sith from nothing which would go on to annihilate the Jedi Order, and rule the entire galaxy save for a select few star systems, instituting possibly the most dominant Sith empire in recorded history. Also played a pivotal role in defeating Abeloth.

Karness Muur - Inadvertently saved Krayt's life from Wyyrlok.

Kol and Cade Skywalker - Through their combined actions, they would topple the Sith Empire led by Darth Krayt, essentially saving the galaxy once again. Kol in particular is responsible for fostering a positive relationship between the remnants of the Yuuzhan Vong and the wider galaxy, and Cade's birth, giving the galaxy potential for a happy ending.

Great list. I'd also add Xesh and Lanoree Brock to that list, as they both saved the Je'daii Order during their adventures, enabling the institution of the Jedi Order. Naga Sadow is also a worthy mention, as his expansionist ideologies led to the Great Hyperspace War, the first conflict between the Jedi and Sith religions that was remembered even 5000 years later. The war also led to the genocide of the Sith people, which allowed for Lord Vitiate to manipulate them into the Ritual of Nathema by playing on their fears, and that chain of events lasted for over 1300 years. Exar Kun should be mentioned too, as it was his fall and the following galaxy-wide conflict started the trend of the unhealthy paranoia about the dark side among the Jedi, leading to the banning of marriage and romance.

You put Muur twice, ILS.

Jump into a volcano

Nearly perfect list from ILS, but I would also add two people.

Yoda - somebody who guided the Jedi Order for literally centuries, trained people who trained people who trained people that were the most crucial Jedi for the galaxy... really has to be on the list, no contest.

Onimi - was behind the most bloody war in the history of the entire galaxy, he has to count.

Odan-Urr, then.

Yeah, Onimi is also a great one. Wasn't sure who to put for Vong.

1. Pall
2. Sadow
3. Tenebrae
4. Revan
5. Outlander
6. Bane
7. Sidious
8. Kenobi
9. Anakin
10. Luke
11. Onimi
12. Krayt
13. Cade

Here's my top 13.

^ is that only in chronological order or also in importance?

Chronological.

Hard to compare the "importance" of, say, Sadow vs Sidious.

Technically Sadow had a larger impact, but Sidious is Sidious.

I mean you definitely need the Exile in, if the Outlander is. Restarting the Jedi order and making the Republic strong enough to survive an assault from the Sith Empire along with freeing Revan so the two of them could scare Vitiate into holding off for centuries is pretty important in the grand scheme of things mmm

I just threw that all under Revan.

The thing about Star Wars is that, like many high-stakes fictional narratives, things are set up so that virtually every single protagonist in the story is absolutely critical to the triumph of the good guys, to the point where there's laughably little margin of error.

Agreed, but in a list like this you can easily narrow down to a list of people who physically had to exist for certain events to take place. Anakin for defeating Sidious, the Outlander for defeating Valkorion, the Exile for bringing down Nihilus etc.

I don’t think Kenobi necessarily fits that role, but that’s more of a philosophical discussion about destiny than about a fictional character’s contribution to the galaxy.